preprocessor: input preprocessing

https://drafts.csswg.org/css-syntax/#input-preprocessing

``` d
import std.stdio;

void main()
{
auto s = "A\u0000B";
s.writeln; // AB
s.length.writeln; // 3
}
```

Start with the linked CSS Syntax specification section on input preprocessing and inspect the parser's existing input-handling entry point. Compare its behavior with the provided D example, then verify that the implementation follows the specification for the shown input.

Written by the indexing model from the issue text.